For coating electric and electronic components, e.g., varistors, composite glass powders are used which, suspended in a suitable liquid, are applied in a layer to the component by spraying with a spray gun or by other suitable processes; and the resultant component is subjected to glaze baking to form a solid firmly bound to the component, e.g., ZnO ceramic in varistors. Composite glass powders are used, since the required mechanical, electrical, and chemical properties of the coating often cannot be achieved with a pure glass powder.
Composite glass powders generally comprise approximately 60-70% by weight of a base glass, especially a lead borate glass; approximately 15-20% by weight of at least one filler to match the expansion coefficient; 5-13% by weight of a compound for achieving specific electrical properties of the coating; and approximately 7-13% by weight of SiO.sub.2 for improving the flow behavior. The base glass forms a solid coating, incorporating the other components therein.
The filler, added in an amount of 15-20% by weight, matches the expansion coefficient of the coating to that of the base. Suitable fillers are, e.g., .beta.-eucryptite, cordierite, mullite, zirconium silicate or other refractory materials, lead titanate, and willemite, as well as materials designated as a "solid solution", such as, e.g., SnO.sub.2 --TiO.sub.2. The fillers can be produced naturally or synthetically. Especially preferred are .beta.-eucryptite and cordierite.
The mixture further contains 5-13% by weight of a compound with which certain electrical properties, e.g., current-voltage characteristics, can be adjusted. Such compounds include, for example, copper carbonate, copper oxide, chromium oxide, silver carbonate, bismuth oxide, magnesium oxide, and manganese oxide.
Especially advantageous is the use of copper carbonate, since the latter decomposes during the coating process under normal operating temperatures to a copper oxide having a very large specific surface; therefore, the resultant oxide can be readily incorporated in the base glass.
As another additive, the composite glass powder also generally contains 7-13% by weight of SiO.sub.2 for improving flow behavior. The SiO.sub.2 can be present in the composite glass powder in the form of quartz powder, as well as in the form of pyrogenic silicic acid.
The base glass powder, as well as the fillers and additives, are present in the composite glass powder in a very fine particle size. The fillers, which are added to match the coefficient of expansion of the coating to the object to be coated, in general to lower the expansion coefficient, normally have a substantially different expansion coefficient from that of the base glass. If the fillers have an excessively large particle size, microcracks in the glazing layer can form, resulting in deterioration of the mechanical and electrical properties of the layer. The fillers used for matching the expansion coefficient, therefore, normally have an average particle size of less than 15 .mu.m. Also, the base glass present in the composite glass powder normally has an average particle size of 4-15 .mu.m, especially a particle size of 6-12 .mu.m; otherwise, the particulate glass is not suitable for uniform incorporation of fillers and additives.
The compounds used for modifying the electrical properties, as well as the SiO.sub.2 added for improving flow behavior, are intended to react during glazing with the base glass, i.e., to be at least partially dissolved in the base glass. The smaller the particle size and the larger the specific surface area of these components, the less the time required for dissolution. Thus, the slower the reaction of these additives with the base glass powder, the finer the additives must be. Silicon dioxide powders dissolve comparatively well so that an average particle size of a maximum of 10 .mu.m can be used. Other additives, such as, e.g., the compounds used for adjusting the electrical properties, especially CuO, may be used only with a maximum average particle size of 4 .mu.m because of their poor solubility in the base glass. Thus, it is further preferred if these slightly soluble additives are added not as oxides, but in the form of compounds which are decomposable below the glazing temperature. During decomposition, such compounds result in very reactive powders having a large specific surface, which greatly contributes to improved solubility of the additives into the base. It is here to be noted that the addition of additives for improving or modifying the electrical properties during the melting step used for the production of the base glass is, in general, not possible, since such an addition results in the separation and crystallization of the base glass.
The decomposable compounds added to the powder in order to modify the electrical properties of the composite are decomposed by heating during the glaze baking step. However, this heating reaction requires time; thus, during the glaze baking step, the product either has to be heated very slowly or a pause in the heating at the decomposition temperature must be employed, thereby extending the baking process. Prolonging the baking process results not only in long cycle times, but can also reduce the quality of the components to be coated.
Another drawback of known composite glass powders is that they must be mixed with a significantly large amount, e.g., 35-40%, of suspension medium, e.g., water, because of the fineness of the base glass powder and additives so as to produce an easily processable, e.g., sprayable, slip. This relatively high proportion of suspension medium results in serious drawbacks. In the drying of the layer applied by spraying, immersion, or other suitable process, a substantial drying shrinkage occurs, and associated therewith is the great danger of crack formation, which results in defective coatings. Further, it is not possible with slips which contain a high proportion of suspension medium to apply sufficiently thick layers, since the layers tend to run.